This invention relates to a method for operating a combination partial oxidation and steam reforming fuel processor.
Fuel cells have been used as a power source in many applications. Fuel cells have also been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a xe2x80x9cmembrane electrode assemblyxe2x80x9d (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer membrane-electrolyte having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distribution of the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode catalysts. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation, assignee of the present invention, and having as inventors Swathirajan et al. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A group of cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture primarily containing O2 and N2) The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies which comprise the catalyzed electrodes, are relatively expensive to manufacture and require certain controlled conditions in order to prevent degradation thereof.
For vehicular applications, it is desirable to use a liquid fuel, such as methanol (MeOH), gasoline, diesel, and the like, as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within the primary reactor of the fuel processor. The primary reactor has a catalyst mass and yields a reformate gas comprising primarily hydrogen and carbon dioxide. A conventional exemplary process is the steam methanol reformation process where methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide according to this reaction: CH3OH+H2Oxe2x86x92CO2+3H2.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. No. 08/975,422 (U.S. Pat. No. 6,232,005) and Ser. No. 08/980,087 (U.S. Pat. No. 6,077,620), filed in the name of William Pettit in November, 1997, and U.S. Ser. No. 09/187,125 (U.S. Pat. No. 6,238,815) Glenn W. Skala et al., filed Nov. 5, 1998, and each assigned to General Motors Corporation, assignee of the present invention. In U.S. Pat. No, 4,650,722, issued Mar. 17, 1987, Vanderborgh et al. describe a fuel processor comprising a catalyst chamber encompassed by combustion chamber. The combustion chamber is in indirect heat transfer relationship with the catalyst chamber and the hydrocarbon is being reformed in the presence of the catalyst.
The indirect heat transfer arrangement between the combustion chamber and catalyst chamber results in extensive time required to heat the catalyst bed to a temperature suitable for fuel reformation. Often, a catalyst regenerating cycle is required to restore the properties of the catalyst after periods of reformation. Therefore, it is desirable to have a method which provides rapid heating of the catalyst beds and timely regeneration of such beds in a reformer.
In one aspect, the invention provides a method for operating a fuel cell system. The system comprises a reactor having one or more catalytic beds and is fed a hydrocarbon fuel along with air and steam. Where more than one catalytic bed is present, such catalytic beds are preferably arranged sequentially such that the outlet from one bed enters the inlet of the next bed. The catalytic beds are the regions where reactions among the hydrocarbon, air, and steam are catalyzed within the reactor. The method comprises supplying a stream of a fuel and air mixture to the reactor which is lean. The mixture is lean in that it has an excess amount of oxygen relative to the stoichiometric amount required for reaction with the fuel. The reactions occurring with the lean mixture heat the reactor. When there is more than one catalytic bed, the hot gases generated from one catalytic bed can be used to heat other or subsequent catalytic beds. When a single bed is used, the hot gases generated at an upstream end of the bed heat the downstream portion(s) of the bed. After sufficient heating of the reactor by the lean mixture, a fuel-rich stream is fed to the reactor. This fuel-rich mixture comprises fuel, air, and water in the form of steam. The mixture is rich in that fuel is fed in an excess amount relative to the amount of oxygen for a stoichiometric reaction. The reactions of the fuel-rich stream produce a product comprising hydrogen (H2). Other typical components of the product stream are carbon dioxide, carbon monoxide, nitrogen, water, and methane.
In another aspect, following the lean fuel-air mixture, a steam stream is fed to the reactor to purge the reactor. Subsequent to the purge, a fuel-rich fuel and air mixture is fed to the reactor along with steam. In one preferred aspect, the first catalytic bed preferentially oxidizes the fuel with oxygen in the fuel/air mixture. The second catalytic bed provides for further reaction and preferentially catalyzes the products from the first catalytic bed with steam for the production of a product comprising hydrogen and other components. In the case where a single bed is used, three main reactions, partial oxidation, steam reforming and high temperature shift occur in the same bed. The regions of the bed over which such reactions occur typically overlap and change with changing power levels.
One of the advantages of this method is the prevention of, or reduction of, carbon formation. Carbon formation tends to degrade the catalyst on the catalytic beds and decrease the reactor""s operating life. Carbon formation also plugs the reactor and decreases flow through one or more of the catalytic beds.